[llvm-dev] [RFC] Removing debug locations from ConstantSDNodes

[Vedant Kumar via llvm-dev]

> On Jun 19, 2018, at 6:36 PM, Reid Kleckner <rnk at google.com> wrote:
> 
> On Tue, Jun 19, 2018 at 5:46 PM Vedant Kumar <vsk at apple.com <mailto:vsk at apple.com>> wrote:
> Someone (Reid?) mentioned that we could try sinking constants to their point of first use as an alternative, and (IIUC) create new nodes with distinct DebugLocs for each use of a constant. I don't recall the details of that alternative clearly. Based on my (likely incorrect) understanding of it, dropping locations from constants outright might be simpler.
> 
> Our use case was in fastisel, so things are different. I don't think my solution will help you.
> 
> In our case, users were complaining about code like this:
> volatile int do_something;
> void f() {
>   ++do_something;
>   foo(1, 2, 3);
>   ++do_something;
> }
> 
> We'd generate locations like:
>   .loc line 1
>   incl do_something(%rip)
>   movl $1, %ecx
>   movl $2, %edx
>   movl $3, %r8d
>   .loc line 2 # line 2 starts here, instead of 3 instructions earlier
>   callq foo
>   .loc line 3
>   incl do_something(%rip)
> 
> Our users really wanted the line table entry for line 2 to include the constant materialization code for some VS debugger feature.

Got it. (For others following along, there's more discussion about this debugger feature in the commit description for r327581.)


> I think if you remove locations from ConstantSDNodes, you might want to add a late pass that propagates source locations backwards onto location-less instructions. This would also avoid some special cases when a basic block starts with an instruction that lacks location information. See CodeViewDebug::beginInstruction and DwarfDebug::beginInstruction for what we do today.

Interesting, I hadn't considered doing this.

What happens in the special case where a block starts with a location-less instruction? Ah, I see that CodeViewDebug::beginInstruction does a forward scan to find the first instruction with location. This can fail though: there might not be such an instruction, in which case... I assume we either apply a line-0 location, or nothing at all, similar to Dwarf::beginInstruction?

It's a bit unclear to me what the benefits of a late pass that backwards-propagated locations would be. I suppose we'd have fewer forward scans in CodeViewDebug::beginInstruction, so is the benefit a compile-time win (which would offset the cost of a late pass)? Would the final debug info quality would be better in some way I'm just missing?

thanks!
vedant
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